Continuing developments in the aerospace industry are allowing many products to become smaller, lighter weight, and operate at much higher speeds and power densities than have be heretofore known. One such product enjoying continued advancement is an electric generator manufactured by the assignee of the instant invention. This advanced generator 100 is a high power, light weight, synchronous two-pole, 24,000 rpm machine whose assembly comprises a main generator 102, a permanent magnet generator 104, an exciter generator 106, and a rotating rectifier assembly 108 as shown in FIG. 1. As the generator is driven by a prime mover, such as an aircraft engine, through a constant speed hydromechanical drive, the permanent magnet generator (PMG) 104 produces a sinusoidal electric output which supplies power to the generator controller 110. The controller 110 rectifies this sinusoidal output to drive the exciter field 112 via a voltage regulator circuit as is known in the art, for example as described in U.S. Pat. Nos. 4,044,296; 4,152,172; 4,245,183; 4,262,242; 4,446,417; 4,477,765; 4,567,422; 4,933,623; or 5,285,147. As the exciter field 112 is energized, it induces a voltage in the rotating armature windings 118 of the exciter generator 106. This poly-phase voltage is then rectified by the rotating rectifier assembly 108 to energize the main rotating field winding 114, which in turn induces an output voltage in the main generator's poly-phase stator windings 120. The level of excitation is varied to regulate this output voltage which is then coupled to the aircraft system electrical loads by the distribution feeders.
The output voltage regulation is typically accomplished by modulating a current signal to the exciter field 112. As a greater system load is connected to the distribution feeders, the duty cycle of the modulation is increased to allow increased excitation to maintain the output voltage at a constant level. If, however, one of the rotating rectifying diodes of the rectifier assembly, e.g. diode 116, were to become shorted, a half cycle phase-to-phase short circuit of the rotating exciter armature windings 118 would occur whenever diodes 122 or 124 are forward biased. A result of this fault is a depressed output voltage because less power actually flows through the main exciter field 114 due to the half cycle phase-to-phase short circuit. In response to the lower output voltage, however, the controller 110 increases the duty cycle of the exciter field drive circuit to increase the level of excitation to the generator 100, and thereby the output voltage. Depending on the system loading of the generator 100, the output voltage may recover to its nominal level, but with a greatly increased exciter current. FIG. 2 illustrates the average exciter field current versus generator load for a normal condition, trace 126, and a shorted rotating diode condition, trace 128. As may be seen from this figure, greatly increased exciter field current is required to maintain output voltage at any given load during the fault condition.
To guard against such faults, a system of protection as illustrated in FIG. 3 is typically employed by the controller 110. This protection monitors each phase of the generator output current I.sub..PHI.A, I.sub..PHI.B, I.sub.101 C, and determines the average generator load current I.sub.L via block 130. The load current I.sub.L is then input to a function block 132 which calculates the expected maximum exciter field current I.sub.EXC(CALCULATED) for the given generator load current I.sub.L (see FIG. 2, trace 126). A logic block 134 then compares this calculated exciter field current to the actual monitored average exciter field current I.sub.EXC(ACTUAL). If the actual average exciter field current exceeds the calculated value based on actual load current (I.sub.EXC(ACTUAL) &gt;I.sub.EXC(CALCULATED)) for a given period of time 136, the controller 110 will de-energize the generator to isolate the fault.
If the shorted diode fault exists upon initial excitation of the generator, however, the normal shorted rotating diode protection may not be capable of sensing the fault due to action of protection circuitry associated with the exciter field driver circuit itself. Prior to initial excitation of the generator, the output voltage is approximately zero volts. Since the typical output voltage regulation circuit is a closed loop system as described above and known in the art, the zero volt feedback upon initial excitation will result in maximum duty cycle of the exciter field driver circuit. Normally, the output voltage of the generator quickly builds, the duty cycle is reduced, and the output voltage achieves and maintains its nominal value. For the condition where the shorted rotating diode exists at initial excitation, however, the peak current flowing is not reduced as with the non-faulted condition due to the half cycle phase-to-phase short circuit of the exciter armature. As the exciter current continues to build, a peak current protection circuit shuts off the exciter driver to protect the circuit from high current stress damage. Since the peak current protection circuit is only concerned with protecting the driver from current "spikes" which exceed its stress rating, it re-enables the driver after a short time, typically within less than a second, to allow continued operation after the transient has passed. If the condition causing the high current is still present, such as in the present case of a shorted rotating diode, the protection will once again turn off the exciter driver once the high current level has been reached, typically in less than 1 millisecond. As a result of this interplay between the latent diode fault and the driver peak current protection, the standard shorted rotating diode protection cannot detect the presence of the fault because the actual average exciter field current does not exceed the calculated exciter field current for longer than the time delay. Standard under voltage protection also does not isolate the fault because the excitation which is provided to the exciter field prior to actuation of the peak current protection circuit is sufficient to induce a generator output voltage which exceeds the under voltage trip limit. Once the peak current protection circuit turns off the exciter driver, the exciter current decays through a flyback diode within the controller. This continued exciter current flow maintains the generator output voltage above the under voltage trip limit for the period until the peak current protection is reset. Once the exciter is re-enabled, the current pulse boosts the generator output voltage once again until the cycle is repeated. The exciter driver continually tries to excite the generator each time its current protection is reset, which results eventually in collateral damage to the generator without indication of the cause of the problem.
Efforts to correct this problem have included oversizing the exciter driver circuit to handle the larger peak currents generated during the initial excitation of the generator during a shorted rotating diode fault condition. Although this technically solves the problem, the use of larger than required drivers prohibitively increases the cost of the controller. Also, the frequency of this type of fault cannot justify the use of the larger, more expensive elements which would be required. Disabling the peak current protection for the driver is likewise not a workable solution due to the damage which may be incurred by the switching elements from such high current spikes.
The instant invention is directed at overcoming these problems without increasing the cost or complexity of the generator controller, nor without disabling the inherent functionality or protections of the standard voltage regulation circuitry known and used within the art for synchronous generator output voltage regulation.